Abstract:
The disjunct zonation pattern of Avicennia marina (Forsk.) Vierh., frequently observed
along the Kenyan coast as a landward and a seaward zone, has been studied on
a morphological level and complemented with preliminary genetic data. The objective
was to describe the two zones in-depth in order to provide a background ecological setting
that may be used in explanations on the emergence of such a bimodal zonation. The
vegetation structure in the two zones is described by means of the Braun-Blanquet relevé
and PCQ-methods, root- and leaf-morphological characteristics, and environmental
variables to provide a background ecological setting. The two zones show considerable
differences in physiognomy of A. marina trees, and indicate that the differences may
constitute an environmental response. Results show that in the landward A. marina
zone, tree heights, leaf sizes, density, and total length of pneumatophores tend to be
smaller as compared to the same attributes of the trees in the seaward zone. Differences
in leaf stomatal density and in above- and below-ground pneumatophore length, the
latter closely related to the effect of inundation, were observed. This suggests a considerable
adaptability and plasticity with respect to environmental conditions. The hypothesis
for the genetic analysis was that mangrove fringes as close as 105 m from one
another constitute a same genetic pool. Preliminary genetic analysis using 48 amplified
DNA fragments (RAPD - Randomly Amplified Polymorphic DNA) from 37 landward
and seaward trees revealed significantly different frequencies for four DNA fragments,
indicating that there might be less contact and genetic interchange between the two
zones than within each zone. Whether this phenomenon, coupled to the different environmental
conditions, can lead to further divergence cannot be answered yet.
